Hello, Currently there is no explicit cfg80211/nl80211 API for triggering a channel survey. It seems that all mainline drivers which implement the dump_survey rdev op manage to gather survey information during a normal scan. While it seems to work perfectly fine in these cases, this assumption cannot be made for all devices. For some context: I am looking into adding survey support for the brcmfmac driver, and the firmware interface has a separate, explicit API for triggering a survey and reading back the results. There appears to be no way to gather survey data through an ordinary scan. Adding a dump_survey op to brcmfmac is not impossible within the bounds of the current cfg80211 API, but it does present some issues. For example, some variants of the out-of-tree bcmdhd driver simply trigger the survey in the dump_survey function. But doing it like this means that a typical survey will hold the RTNL mutex for about 5 seconds - yikes! So my question is: could the API be extended to support explicitly triggering a survey? And if so, how should it look? The original commit (61fa713c7516 "cfg80211: return channel noise via survey API") introducing the dump_survey API notes: > In future, there will either be a survey-trigger command --- or the existing > scan-trigger command will be enhanced. This will allow user-space to > request survey for arbitrary channels. The easiest solution would be just to add an attribute to CMD_TRIGGER_SCAN - say NL80211_ATTR_SCAN_SURVEY - to indicate to the driver that it should collect survey information if possible. Existing drivers would be unaffected (since they already do this implicitly) while a driver like brcmfmac could tell the firmware to do an explicit survey instead. The only issue I see with this is that there would be no new scan results to send back. I wonder if that matters? Otherwise a new CMD_TRIGGER_SURVEY could be added, and the driver could emit CMD_NEW_SURVEY_RESULTS when the survey is complete. I guess the implementation here would look a lot like what we already have for scanning. I'm not sure how far to take this though - should cfg80211 also cache the survey results like it does for scans? In that case it might also require changes to existing drivers which currently populate the survey dump on demand. I'm quite happy to send some patches for this and I just wanted to get some feedback first. I'm less confident about implementing the latter approach, particularly if it involves sending patches for drivers I cannot test (and for a rather niche feature at that). But if I can get some more clarity on what such an interface should look like, I'll see what I can do. Thanks in advance. Alvin
